Chapter 9 Complex Ground Beneath the Tung Chung Reclamation

Chapter 9 Complex Ground Beneath the Tung Chung Reclamation Introduction This chapter describes the geology associated with complex ground conditions at the Tung Chung reclamation. These complex ground conditions were first encountered during preliminary ground investigations for the Tung Chung reclamation in 1991 and have had a significant impact there on the design and construction of deep foundations for infrastructure development. Much of the information presented here is taken from a report entitled “Geology of Tung Chung New Town" produced by the British Geological Survey (Gillespie et al.， 1998) under a Consultancy Agreement managed by the Geotechnical Engineering Office for the Territory Development Department. This detailed study was prompted by the need to establish a reliable geological model to assist further development in the area. The geological model developed for Tung Chung has emphasised how the combination of several distinct geological processes， of widely differing age， has led to the development of the complex ground conditions. Useful related information is included in papers by Kirk (2000)， Kirk et al. (2000)， and Fletcher et al. (2000). Background Prior to its recent development， marble was unknown in the Tung Chung area. The nearest sedimentary rocks had been mapped at The Brothers islands， and marble was proved in offshore boreholes nearby and at Tai O. These marble occurrences had both been correlated with the marble-bearing Carboniferous sequence in the Yuen Long area. Skarn had also been reported from San Shek Wan and Sha Lo Wan (Langford et al.， 1995). The solid geology in the vicinity of Tung Chung was known to be dominated by granitoid dykes (feldsparphyric rhyolite， quartzphyric rhyolite， and microgranite)， with some small intervening slivers of medium-grained granite (GEO， 1994). According to GEO records， marble was first reported at Tung Chung reclamation in late 1992 during drilling at Housing Development Phase 1 ('Area 10'). Consequent geophysical surveys， undertaken by GEO during 1993， suggested that sedimentary rocks occurred offshore to the north of Lantau Island. However， the results for the existing reclaimed area remained inconclusive. In 1994， fragments of marble and small voids were noted from several boreholes at the Pedestrian Bridge site in Tung Chung Phase 1. In 1996， drilling for Tung Chung Town Lot 3 ('Site 3'， Figure 13) encountered a range of problematic geological conditions including marble with cavities， steep weathering profiles reaching to depths of more than 170 m， and unconsolidated sediment to depths of at least 80 m. The complex geological conditions at Site 3， eventually led to the abandonment of a planned residential tower block in late 1996. The preliminary geological model suggested that the marble and associated metasedimentary rocks occurred as small slivers within a fault zone (GEO， unpublished correspondence) although the scale and extent of the problematic ground conditions were unclear. Attempts to understand the geological controls on the distribution of the unusual materials were hindered at the time by inconsistent description and interpretation in borehole logs undertaken by different contractors. This made correlation or comparison between adjacent sites very difficult. Tung Chung Study The Geotechnical Engineering Office， on behalf of the Territory Development Department (TDD)， and assisted by British Geological Survey consultants， carried out a geological study of the entire Tung Chung New Town area between 1997 and 1999. The objectives of the study were to collate all available ground investigation (GI) data; develop a geological model for the occurrence of complex geological conditions; provide guidance on the most appropriate GI techniques to enable early recognition and effective delineation of affected areas; and to evaluate the possibility of other areas being similarly affected. Follow-up studies， based on 48 ��e �� at�� a� �� a�t �� t�e �� a�t�e�la�at��te�t�all��e�la��le� ��t�e�� at��a��a�t ��e 812 500E812 500E 812 500E812 500E 813 500E813 �e�l��al��t�� 817 500N 817 500N -60 -60 -100 -100 20 20 -80 -80 -60 -60 40 40 -40 -40 -100 -100 -120 -120 -60 -60 -140 -140 -80 -80 -40-40 ry Pier -20 -20 0 0 0 HIGHWAYScale Scale 200 m 200 m 49 -40 -40 100 100 0 0 Legend Legend 817 000N 817 000N LANTAU -40 -40 Marble, limestone or other marble and limestone-bearing sedimentary rocks, and their weathering products A (particularly cavity-fill deposits and residual soil) IR NORTHBoreholes proving marble, limestone or marble-and PO limestone-bearing sedimentary rocks RT Boreholes proving anomalously deep (below -50mPD) -20 -20 RA rockhead in igneous rocks T T I C LW C AY Subcropping soft to stiff siltT or fine alluvial sediment which may lie above areas affected by marble-related rocks or anomalously deep rockhead in igneous rocks ( based on CHUNGTUNG TUNGCHUNG borehole, gravity and shallow seismic data ) Boreholes proving soft to stiff silt or fine alluvial sediment Chek Lap Kok 00 00 below -50mPD Proposed Reclamation -20-20 -20 -20 Site 3 Site 3 Contour of rockhead (20m intervals) (mPD) ung Chung Lantau Island Crescent recommendations contained in the report (Gillespie et al.， 1998) included: further onshore and marine ground investigation to test models and investigation techniques， onshore and marine geophysical surveys， and development of a database for all archival and new information. Data Sources Some 2，500 boreholes have been drilled in the Tung Chung New Town area over the last 20 years. Most have been carried out within the last few years as the tempo of development has increased. Despite the volume of information available， there were problems in developing a geological model for the site. Problems arose mainly from the inaccurate or inadequate descriptions of geological materials， and widely differing interpretations of some of the less common lithologies. For example， the various logging geologists preparing the numerous GI reports from areas of the reclamation adopted a wide range of interpretations of the highly varied lithologies present. In some instances， several versions of logs existed for the same hole， probably reflecting the difficulties that contractors and consultants were experiencing while attempting to interpret the unusual materials (e.g. Figure 14). Logging problems were further compounded by sampling strategies. In most cases the very thick soils at the site were only logged from Standard Penetration Test (SPT) and Mazier cutting shoe samples. Numerous Mazier samples were reserved for future laboratory testing， but very few were used to help resolve the difficulties encountered in lithological description. Thus， it was not possible to build up a consistent geological model from the widely differing information， ranging across many sites. As part of the GEO study， logs (and particularly core photographs， where available) of about 2000 boreholes were reviewed. Consistent terminology and classification were applied to a revised set of descriptions. About 400 boreholes were completely re-logged by re-examination of some or all of the samples from each hole. However， it should be noted that not all the borehole logs have been reviewed by GEO. The re-logging undertaken in the GEO study is now stored digitally in Planning Division's Geological Modelling System. The original paper records， most of which are stored in the CEL GIU， have not been updated. Future workers accessing “factual data" for the Tung Chung reclamation from the GIU should therefore be aware of the difficulties of using the original paper records as outlined above. Developing the Geological Model The geological model developed from the reassessment of borehole logs during the Tung Chung study is described below in terms of three stages: a) a description of the metasedimentary xenoliths， and their inclusion into the local granitic rocks; b) a description of the deep weathering of the granitic rocks in the vicinity of some of the xenoliths; and c) a description of the development of karstic features in and above those xenoliths that contain carbonate rocks. Xenoliths The area is underlain by granite， intruded by numerous granitoid dykes (mainly feldsparphyric rhyolite). The intrusive rocks enclose blocks (xenoliths) of metasedimentary rocks， predominantly of marble， sandstone and siltstone. The blocks range in size from a metre across to more than 300 m across. It is inferred that the blocks were incorporated during (Jurassic) intrusion， as they commonly exhibit effects of contact metamorphism. For example， skarn is well developed where carbonate rock (such as marble) has been in contact with the magma. The structural geology of the area is complex， and faulting further complicates the original distribution of xenoliths. Weathering Extreme depths of weathering are evident where the sedimentary rock xenoliths occur. In the Tung Chung area， continuous Grade III material and better， typically occurs at depths of 40 to 50 m. However， in the immediate vicinity of some xenoliths， many boreholes have been drilled to 150 m or more without reaching continuous Grade III or better material granite， and at least one borehole exceeds 200 m. These unusual weathering profiles are characterised by abrupt weathering fronts with transitions from Completely Decomposed Granite (Grade V) to Slightly or Moderately Decomposed Granite (grades II， and III) over only a few metres. The gradient on these transitions， also referred to as

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